1. Field of the Invention
This invention relates to apparatus for maintaining tension in a drive belt and for absorbing the shock occurring when a carriage driven by the drive belt is driven to an end of its travel, and, more particularly, to maintaining tension in the drive belt of a point of sale printer and to absorbing the shock of a print carriage being driven to its end of travel.
2. Description of the Related Art
Point of sale print terminals are widely used to print sales receipts, credit card receipts, and to print journal tapes including the data for a number of transactions. Such terminals are also increasingly used to print information, such as franking information on checks offered by customers for purchases.
A typical point of sale print terminal includes a wire matrix print head, which is mounted on a carriage driven horizontally, along the paper on which printing is to occur, by means of a motor driving a belt attached to the carriage. For successful printing, the printer electronics must have accurate information concerning the horizontal position of the print head. Such information is necessary to place the characters being printed in the appropriate positions on the paper and to form the individual characters themselves with a wire matrix mechanism having, for example, a single vertical line of printing wires within the print head.
In a typical point of sale print terminal, the motor used to drive the carriage is a stepper motor, and the belt drive extending between the motor and the carriage includes a toothed pulley driving a toothed belt, so that slipping does not occur between the motor and the carriage. During operation of the printer, location data providing the printer electronics with the location of the print head is derived from the electrical signal used to drive the stepper motor. After each line is printed, the print head is driven to a home position, in which a transducer provides an accurate and reliable indication that the print head has reached a particular point. This indication may be used, if necessary, to reset the location data generated using the stepper motor drive signal to a position corresponding to the home position, and to generate an error signal if the home position is not reached when it should be. The home position is typically provided at one end of the travel of the print head and carriage, so that, after every other line of printing, occurring as the print head is driven in each direction along the document being printed, the print head is driven to the home position.
Various conditions can effect the operation of this type of printer so that the data concerning the location of the print head is lost, or so that this data becomes inaccurate. These conditions include the loss of electrical power to the print terminal, manual movement of the carriage, or some obstruction is encountered. When the carriage reaches the home position, whether or not such a condition has occurred, it is stopped, with the information that it has reached this position being provided to the printer electronics. If the printer electronics determines that the data concerning the location of the print head has been lost, the print head is driven to the home position, to be stopped when the transducer indicates that this position has been reached.
Many point of sale terminals have dual print stations configured to print two separate documents extending within the printer in a spaced-apart relationship. For example, the terminal may be provided with two rolls of paper, so that sales receipts are printed on a first roll of paper for presentation to the customers, while a journal is printed on a second roll of paper for subsequent use by the store to recover sales information. This arrangement allows the sales receipts and the journal roll to have different printed information. For example, the sales receipts may include spacing to facilitate the separation of sequentially printed receipts, with information identifying the store, and even advertising messages, together with the identification of various purchases, while the journal roll has sales information printed in a much more compact form. In a high-volume store, the resulting savings in the length of the journal roll, compared to the sales receipts, is significant.
In order to minimize the time required to check out purchases, it is particularly desirable to minimize the time required to print both the sale receipts and the journal information. The rolls can be printed together, with the print head being moved across both rolls to print each line, or they may be printed sequentially, with the journal data being printed after each corresponding sales receipt. Alternately, the journal data corresponding to a number of sales transactions, for which data has been stored within a computer system, may be printed on the journal roll at a convenient time.
Because this kind of operational flexibility is important, a point of sale terminal having dual print stations should operate efficiently when printing on the two rolls together, or when printing on either roll separately. The use of a single home position at one end of the carriage motion, configured in the manner described above for a point of sale terminal having a single print station, is undesirable because, whenever the print station opposite the end of travel at which the home position is located is used, much additional time is required during the printing operation to go to the home position and to return to the print area.
One solution for this problem is to provide a home position at each end of the carriage motion, so that the adjacent home position can be used whenever only one of the print stations is being used. The disadvantage of this solution arises from the cost and complexity of the additional hardware needed, such as an additional position sensing transducer.
Another solution for this problem, which is described in the IBM Technical Disclosure Bulletin, Vol. 34, No. 5, October, 1991, pp. 462, 463 is to provide three flags, disposed along the drive belt to be sensed by a single position sensing transducer, with the spacing among the flags being such that a determination of the position of the print head can be made by counting the number of pulses to the stepper drive motor required to drive the print head through the distance between flags. Again, the disadvantage of this solution arises from the cost and complexity of the additional hardware needed, such as flags attached to the drive belt.
Another solution to this problem is to provide a single, central home position between the two printing fields of the printer. However, if the location data is lost, there is no way of determining the side of the central home position without moving the carriage. That is, there is no way to determine the direction to move the carriage to encounter the home position. For example, if the carriage is initially moved to the left, the home position will be encountered in the event that the carriage starts at the right of the home position, but the left end of travel of the carriage will be encountered first if the carriage starts at the left of the home position. When the end of travel position is reached in this way, the motor continues to be driven without further carriage movement. However the printer electronics drives the motor only with a maximum number of pulses, which must be sufficient to move the carriage in the left direction to the home position from wherever it is initially located on the right side of the home position. When this maximum number of pulses is exceeded, with the carriage being held against its left end of motion, the motor is driven to move the carriage to the right, until the home position is reached.
A problem with this last solution arises from the fact that a loud impact noise occurs as the carriage is moved to its end of travel position. This noise is sufficient to create a suspicion that something is wrong with the printer terminal. Thus, what is needed is a way to prevent the kind of impact causing such a noise, while allowing the carriage to be driven into its end of travel position after the carriage location data is lost.